In analog control systems, there may be a need to allow different circuit paths to set a control voltage at a common output at different times. Therefore, there may be a need for a circuit that can detect the control voltages of each circuit path to determine an overall control voltage. For example, in negative feedback systems, the overall control voltage may be set to a minimum voltage from among multiple control voltages. A maximum voltage may be used in other applications.
As an example application of an analog control system, in battery conditioning systems, two separate circuit paths may be used to control a power converter that charges and discharges a battery. One loop may be a current-controlled circuit path while the other may be a voltage-controlled circuit path. During a charging process, these loops may compete to control the battery's charging current by trying set the power converter's control voltage. A minimum selector circuit may select which circuit path controls the power converter.
A variety of minimum selector circuits or maximum selector circuits are known. A conventional minimum or maximum voltage detecting circuit may have the drawback that, the transfer function from any input of the circuit to the output cannot be easily altered. As such, any different transfer function may need to be implemented with additional circuitry, thus increasing the complexity and cost of the overall circuit and system.
Some circuit architectures may have long transition times. They may have intermediate outputs that are driven to the supply rail (+/−15V, for example), which increases the transition time required to switch from a previously output of a minimum or maximum voltage to a new output minimum or maximum voltage. Increases in the transition time may cause voltage errors at the output of the selector circuit which can degrade the performance of the overall system.
Thus, there may be a need for new minimum or maximum voltage detecting circuits and systems that can implement any transfer functions in their feedback circuit, while having low transition time in switching and better overall speed performance.